RE: Flow rate if Oroville Dam aux spillway fails?

RE: Flow rate if Oroville Dam aux spillway fails?

The limiting case would be an instantaneous failure, which could be estimated by the basic weir equation. In reality, it would take some time for the breach to develop, so the actual flow would be somewhat less. A dam breach/break analysis would be necessary to evaluate a progressive failure.

RE: Flow rate if Oroville Dam aux spillway fails?

Also, in looking at some of the photos and video from today; it appears that the failure would most likely occur through the right abutment. I doubt there is a dam break analysis for that case that has been ran. However, a dam break analysis of the embankment would be more conservative than one through the abutment.

All in all, looks like it is going to be hard to save the existing spillways and if the erosion starts moving toward the embankment, the whole thing could go. Quite a mess, will be interesting to see why they think the spillway slab failed.

Mike Lambert

RE: Flow rate if Oroville Dam aux spillway fails?

the dam will not fail. The headcut will proceed back to the control section and according to data I have seen that would be 30 feet or so below the maximum elevation of the reservoir. The width of that headcut would gradually widen, depending on the erodibility of the ground.

RE: Flow rate if Oroville Dam aux spillway fails?

That sounds right. I did you the weir equation to get a ballpark of absolute worst case scenario - over 400,000 cfs. I think it is more reasonable to think worst likely case is more like 300,000 cfs - which is still double the capacity of the downstream levees.

Thanks, Steve

RE: Flow rate if Oroville Dam aux spillway fails?

Thank you for those estimates. 640,000 cfs is a frightening number. Fortunately virtually impossible for the conditions to set up that way. Half of that is still a devastating flow for the region - likely inundate well over 150 sq miles, tens of thousands of homes.

Thanks again.

RE: Flow rate if Oroville Dam aux spillway fails?

I did not do any failure analysis since I have no data. But, obviously the water level would drop rapidly before a full spillway head-cut breach would develop, so the maximum rate would never happen. And assuming there is some bedrock in there, the head-cut might not ever develop fully as assumed. Also, the weir equation might be overly conservative, there could be some tail-water effects. Either way, it illustrates the magnitude which could be enormous. The inundation would need to be mapped with 2-dimensional analysis. Something like FLO-2D might be used to evaluate the actual inundation limits.

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